1. Field of the Invention
The present invention relates to dinuclear and water-soluble rhodium complexes, and to the use of such complexes or aqueous solutions thereof as hydroformylation catalysts.
2. Description of the Prior Art
Dinuclear rhodium complexes of the type (1): ##STR2## and their use as a hydroformylation catalyst are described in U.S. Pat. No. 3,501,581. However, these catalysts have low activity.
Dinuclear rhodium complexes of the type (2): ##STR3## (in which t-Bu denotes a tert-butyl radical) or of the type (3): ##STR4## denotes a residue derived from chloromethylated polystyrene: ##STR5## and their use as hydroformylation catalysts are described in U.S. Pat. No. 4,215,066.
However, while the fundamental interest in these rhodium complexes is not in doubt, the development of their use as catalysts on an industrial scale is hindered by their lack of selectivity. In fact, the ratio n/n+iso (n: normal linear aldehyde; iso: branched aldehyde) which complexes of this type provide in hydroformylation reactions is notoriously inadequate, which severely burdens the overall economics of this process.
Furthermore, the recovery and the recycling of the complexes of the type (2) are complicated by difficulties traditionally associated with homogeneous catalysis reactions, difficulties whose severity is further increased by the fact that the loss of even the smallest quantity of rhodium is economically prohibitive.
It might have been thought possible to remedy these disadvantages, at least insofar as the catalyst recovery and recycling are concerned, by using complexes of the type (3) above, namely, compounds in which the organometallic complexes are "immobilized" on a solid organic (or mineral) support through the intermediacy of functional groups in the said support. It is generally observed, however, that a significant part of the metal goes into solution in the liquid reaction phase (see, particularly, W. H. Lang et al, J. Organomet. Chem., 134, 85 (1977)).
In addition, as taught by J. Falbe in New Synthesis with Carbon Monoxide, Springer Verlag (1980), the catalyst solids thus obtained are much more sensitive to deactivation and poisoning than similar soluble catalysts.
It will be seen, therefore, that the merit of such "supported complexes" cannot be established industrially, all the more so since their use involves difficulties in connection with mass and/or heat transfers.